This invention relates to a method for gas metal arc welding with an inverter power source, and a welding power source of the inverter type with properties associated with terminating welding which prevent the welding wire from sticking in the contact nozzle on the welding burner connected to the welding power source.
In gas metal arc welding, in the following called MIG/MAG welding, the equipment comprises a gas container, reel of wire, wire feeder, welding power source, welding cable and welding burner which comprises a first pipe for taking the protective gas to the molten pool and a second pipe situated inside the first to take the welding wire to the molten pool and to transmit the welding current to the said welding wire. In the following this second pipe is called the contact nozzle.
There has always been a certain tendency for the wire to stick in the contact nozzle. The solutions have concentrated upon reducing the friction in all parts which take the wire from the wire feeder to the workpiece. Wire guides of Teflon or similar material are an example of such a solution, and great care has been taken to manufacture the wire with as smooth a surface as possible.
Other measures have regarded the main problem as being deposits from the welding wire, either from its thin copper layer or from residues from the wire drawing process.
Solutions have been of a mechanical naturexe2x80x94better feeders, greater tolerances between the wire and the hole in the contact nozzle or self-cleaning contact nozzles.
However, wider tolerances in the contact nozzle result in a poorer electrical contact and thereby poorer current transmission.
This has been solved by ingenious contact nozzles such as spiral-shaped or parrot-beaked nozzles in order to achieve better contact points. Contact nozzles of the normal pipe model have sometimes been given improved characteristics by a little mechanical impact in order to create a distinct contact point. However, such solutions result in increased friction which is not desirable for even wire feeding. In addition, the mechanical wear and tear on such nozzles is greater, which reduces their working life.
Since the 1980""s welding power sources of the inverter type have been used. The weight and size of the power source is reduced, which is an advantage. In particular, however, the power sources are much quicker to regulate which has meant that the emphasis for the regulation of the welding process has moved from the wire feeder to the power sources. During continual welding the object of the regulation is to supply melting power at a corresponding rate to the feed rate of the electrode.
Problems associated with commencing welding have been observed with these power sources, particularly when welding with stainless electrodes. The wire can then stick in the contact nozzle at the actual moment of commencing welding. In the work leading up to this invention it was also studied how the wire behaved during the concluding stage, that is just when the welding current is about to be shut off.
The surprising result of these investigations was that the wire did not become wedged in the contact nozzle in connection with the wire feeder motor coming to a stop. On the contrary, at the moment the welding current stopped, the wire was brought to an immediate stop in the contact nozzle, while the wire feeder fed out further wire which was taken up by the play in the wire guide which connects the feeder and the contact nozzle.
The aim of the invention is to produce a method or a welding power source of the inverter type, which provides a solution to the problem of electrodes sticking in the contact nozzle during MIG/MAG welding without the disadvantages from which the known technique suffers.
The invention solves the problem in the way specified in the characteristic portions of the independent claims.